Hi there! First post here. While I'm still gathering materials and working on my initial layout, I really want to talk about just how incredibly useful ebay has been for modeling stuff. Top buys so far for me have been:

Arduino compatible & H-bridge (£2.50 + £1) Power+programming for the train. You can connect the H-bridge directly to the Arduino +5v, and get 4.5v out in both directions, and get both power and steering directly from your computer

1:400 Chinese modeling trees (£3 for 50) and foam (£6 for 250g). Compared to woodland scenics, the shredded foam making up these is significantly leafier and finer. The wire structure of these trees are only joined in one place, the base, so if you want saplings, you can just snip the base off and separate the branches.

0.5mm Carbon fiber rod(£2.50 for 3m) I use this for poles and girders.

120µm shot blasting abrasive(£4.50/100g) I use this to produce a structured grass effect on terrain (I mix it into green paint until it has the consistency of sugar cookie dough, spread it on with a palette knife, and then paint it with water to smooth it out)

5x50ml atomizer bottles(£1) I fill these with various mixes of glue/paint/water)

No-name Plasticine(£3.50/500g) I use this for terrain smoothing on top of the foam structure